The present invention relates to cryopumps of the type having condensation surfaces disposed in their housing and a shield against heat radiation disposed in their inlet region of the pump.
Cryopumps of this type which have substantially axially symmetrical configurations, are disclosed in Austrian Pat. No. 242,409 and German Offenlegungsschrift [Laid Open Application] No. 1,938,035. Their pumping action involves condensation of the gases being pumped on cooled surfaces. The lower the temperature selected for the condensation surfaces, the better is the pumping effect. The amount of energy required to produce the cold temperatures, however, significantly increases when lower temperature values are sought because of the greater losses resulting from heat conduction and heat radiation reaching the condensation surfaces. It is therefore known to provide cooled shields to protect against heat radiation.
These shields for the most part are designed as closed jackets. However, in the entrance region of the pump, the shield must be designed to enable the gases which are to be withdrawn to enter the pump.
Therefore, the shielding in this region must meet two mutually contradictory requirements. On the other hand, it must obturate the condensation surfaces as completely as possible so that no heat radiation can go directly from the receptacle to be evacuated to the condensation surfaces; on the other hand, the shield should interfere as little as possible with the flow of the gases from the receptacle to the pump. At the pressures that can be realized with cryopumps the known radiation shields, which have an axially symmetrical configuration adapted to the likewise axially symmetrical condensation surfaces, constitute a constriction, or bottleneck, which substantially adversely influences the suction capability of the pump.